Gas turbine engines typically include a fan delivering air into a compressor. The air is compressed in the compressor and delivered into a combustion section where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine blades, driving them to rotate. Turbine rotors, in turn, drive the compressor and fan rotors. The efficiency of the engine is impacted by ensuring that the products of combustion pass in as high a percentage as possible across the turbine blades. Leakage around the blades reduces efficiency. Thus, a blade outer air seal (BOAS) is provided radially outward of the blades to prevent leakage.
The BOAS is spaced from a radially outer part of the blade by a tip clearance. The BOAS is associated with a carrier element that is mounted to a case structure. Since the blades, the BOAS, and structure that support the BOAS are different sizes and/or are formed of different materials, they respond to temperature changes in different manners. As these structures expand at different rates during heating, the tip clearance may be reduced and the blade may rub on the BOAS, which is undesirable.
Clearance control systems are used to control the tip clearance under different operational conditions. These systems can comprise active control and/or passive control systems. The systems have a predefined limiting clearance condition that sets all other clearances throughout the flight envelope. Further reducing these clearances to be lower throughout the flight envelope helps meet increasing engine efficiency demands.